Previous studies in 6 males and 7 females (ages 18-37) with Prader-Willi syndrome showed hypercalciuria and reduced bone density of the lumbar spine and femur. To determine the role of gonadal steroids in bone loss, bone density will be measured in 20 subjects with Prader-Willi syndrome treated with depo-estradiol (females) or testosterone cipionate (males) injection monthly for two years.